1. Field of the Invention
The present invention relates to a communication system for controlling a transmission power of a downlink control signal, a base station, a radio network controller, and a transmission power controlling method.
2. Description of the Related Art
A phenomenon of fluctuation in a receiving condition of a data signal received by a mobile station or a base station (such a phenomenon is so-called a “fading phenomenon”) may occur in the radio communication, and occurrence of a fading phenomenon may lead to substantial deterioration in the transmission quality (namely, characteristics including a bit error rate and the like).
Meanwhile, a technique called “transmission diversity control” is generally known to be a method for reducing deterioration of the transmission quality attributable to occurrence of a fading phenomenon. Now, “closed loop transmission diversity control” a type of the “transmission diversity control” will be described below.
Specifically, a transmitter side (hereinafter referred to as a base station) generates downlink data signals and downlink control signals for two lines (hereinafter collectively referred to as downlink signals) and common pilot signals for the two lines, and then generates multiplexed signals for the two lines by multiplexing the downlink signals with the common pilot signals. Then, the base station transmits the multiplexed signals for the two lines to a receiver side (hereinafter referred to as a mobile station) through two antennas.
At this time, the base station multiplies the downlink signals by complex weights corresponding to the respective downlink signals based on feedback information (hereinafter abbreviated as FBI) received from the mobile station, and spreads the downlink signals that are multiplied by the complex weights. Here, the downlink data signals are carried by a downlink dedicated physical data channel (DPDCH), while the downlink control signals are carried by a downlink dedicated physical control channel (DPCCH).
Meanwhile, the base station spreads the common pilot signals for the two lines by use of an identical spreading code. Here, the spreaded common pilot signals are configured to be mutually orthogonal and are carried in an identical carrier phase by a downlink common pilot channel (CPICH).
On the other hand, the mobile station despreads the common pilot signals for the two lines transmitted through the two antennas, and calculates a carrier phase difference between the common pilot signals for the two lines. Then, the mobile station generates the feedback information (the above-mentioned FBI) for adjusting a carrier phase difference of the downlink signals based on the calculated carrier phase difference between the common pilot signals. Here, the FBI is carried by an uplink dedicated physical control channel (DPCCH).
In this way, the base station reduces deterioration in the transmission quality attributable to occurrence of the fading phenomenon by adjusting the carrier phase difference of the downlink signals, which are transmitted through the two antennas, based on the FBI received from the mobile station.
Here, there is a possibility of an error of the FBI in a radio section. If the base station adjusts the carrier phase difference of the downlink signals based on the erroneous FBI, the “closed loop transmission diversity control” will not function effectively.
A technique called antenna verification is known in this regard. In the antenna verification, the mobile station estimates the carrier phase difference of the downlink signals based on dedicated pilot signals which are transmitted through the downlink dedicated physical control channel and the common pilot signals.
In this way, even if there is an error of the FBI in the radio section, the mobile station can decode the downlink signals accurately by estimating the carrier phase difference of the downlink signals in accordance with the antenna verification and performing demodulation and decoding of the downlink signals by use of the estimated carrier phase difference (3GPP TS25.214 V6.4.0).
As described above, the downlink control signals (such as the dedicated pilot signals) transmitted through the downlink dedicated physical control channel is used for the antenna verification that complements the closed loop transmission diversity control.
Therefore, in order to allow the antenna verification and the closed loop transmission diversity control to function effectively, it is preferable to apply large transmission power to the downlink control signals that are transmitted through the downlink dedicated physical control channel.
In the meantime, power resources have a limitation in the radio communication. Accordingly, it is unfavorable to apply large transmission power excessively to the down link control signals that are transmitted through the downlink dedicated physical control channel.